1. Field of the Invention
Embodiments of the present invention relate to techniques for performing reliability testing in computer systems. More specifically, embodiments of the present invention relate to a method and an apparatus for performing swept-sine testing within a computer system to assure vibrational integrity.
2. Related Art
Some computer systems and storage arrays are adversely affected by vibration of system components. These vibrational problems are becoming increasingly more common because of the following trends: (1) cooling fans are becoming powerful; (2) chassis and support structures are becoming weaker because of design modifications to reduce cost and weight; and (3) internal disk drives, power supplies, and other system components are becoming more sensitive to vibration-induced degradation. For example, hard disk drives (HDDs) are becoming more sensitive to vibration because the storage density for HDDs has increased to the point where a write head has to align with a track which is less than 20 nanometers wide. Moreover, the write head floats only 7 nanometers above the disk surface. These extremely small dimensions make the read and write performance of the HDDs very sensitive to vibrations.
Complicating this issue, some computer systems and storage arrays do not lock fan speeds at fixed number of revolutions per minute (RPM). Instead, the fan speeds can vary. For example, at higher altitudes, where the air is thinner, fan blades turn faster. In fact, between sea level and 10,000 ft, fan speeds can vary by 10% or more.
These fan speed variations can cause vibrational resonances inside the chassis of a computer system. When the fan speed intersects an internal vibrational resonance, there can be a significant resonance-related amplification of the vibrations which can cause components such as disk drives and power supplies to fail.
Because fan speeds can vary with altitude, a system that is tested and qualified in a lab at one altitude may get distributed in the field at another altitude where the fan speed intersects a vibrational resonance, which can cause failures. A related problem is that new systems tend to have very low rotational friction for fan motors. With age, ball bearings lose roundness, lubrication dries out, and shaft axes gain eccentricity. Consequently, rotational friction increases and fan speeds can drop with age due to the increase in rotational friction. These effects mean that even if a system is qualified to have good vibrational integrity when the system is manufactured, if there are any structural resonances in the vicinity of the rotational frequency of the fans, the system may subsequently fail in the field.
In order to avoid the damage that can be caused by vibration, some system designers test prototypes of computer systems and storage arrays with a technique called “swept-sine” testing. Swept-sine testing is common for safety-critical mechanical systems such as aircraft and nuclear power plants, where the presence of structural resonances can have catastrophic consequences. To examine computer systems or storage arrays for the presence of structural resonances, system designers typically bolt a system under test onto a large programmable “shake table” that inputs a vibrational stimulus at a fixed frequency. The system designer then “sweeps” this frequency from a starting frequency (e.g. 1 kHz or 10 kHz) down to a very low frequency. By monitoring one or more vibration sensors (accelerometers) placed on or inside the system under test, the system designer can “map” the frequencies corresponding to vibrational resonances inside the system.
When the frequencies of the resonances are known, the system designer can adjust the fan speeds (and the speeds of other devices such as disk drives, tape drives, etc.) to avoid vibrations in the vicinity of the resonances. Alternatively, the system designer can mitigate the resonances. For example, the system designer can place a small mass, a dampener, or a stiffener at a specific location in the system.
The difficulty with the above-described approaches is that a customer may reconfigure their computer system, by adding and/or removing one or more system components. This changes the mass distribution of the system, which can create new structural resonances that can cause drive failures and can accelerate degradation of other system components.
For many computing systems, especially midrange and high-end systems, it is impossible to anticipate all the potential combinations of vendor and 3rd-party components that customers may install over the life of their computer system. Consequently, in order to accurately determine if new vibrational resonances are introduced in the system when a customer makes a configuration change, the system needs to be shipped to a facility with a shake table and retested.
Hence, what is needed is a method and apparatus for performing vibrational testing without the above-described problems.